Variable displacement pump



Jan. 9, 1962 s. G. WILLIAMS 3,016,018

VARIABLE DISPLACEMENT PUMP ATTORNEYS Jan. 9, 1962 s. G. WILLIAMS 3,016,018

VARIABLE DISPLACEMENT PUMP Filed Sept. 17, 1959 2 Sheets-Sheet 2 INV ENTOR S amuel @.Wl ams BY if i527@ l ATTORNEYS United States Patent O Jersey Filed Sept. 17, 1959, Ser. No. 840,643 2 Claims. (Cl. 103-37) This invention relates to variable displacement hydraulic pumps of the type in which displacement is varied in inverse relation to discharge pressure.

Pumps of this type are common. They usually include an adjustable element which is shiftable between minimum and maximum displacement-establishing positions for varying the displacement of the pump, a spring biasing that element toward its maximum displacementestablishing position, and a pressure motor having a Working chamber and a piston which is subject to the pressure in that chamber and arranged to shift the adjustable element toward its minimum displacement-establishing position against the bias of the spring. The working chamber is connected with the discharge port of the pump and, in order to damp high frequency pressure pulsations, this connection is provided with a flow restrictor.

It has been found that when these pumps are used in a tight system, i. e., one having a very low rate of leakage, and demand for hydraulic fluid drops rapidly to zero, the time lag between this change and corrective action on the part of the displacement control mechanism is so great that the discharge pressure overshoots the maximum setting of the pump. In some cases where the setting of the pump was 3000 p.s.i., pressure surges up to 5000 p.s.i. were encountered. Since the leakage rate from these systems can be as low as 5 to l0 cc. per minute, the oversurge condition may last 5 to 30 seconds. Obviously, this is a serious disadvantage.

The object of this invention is to provide a mechanism for Varying pump displacement in which both the magnitude and the duration of the surge pressure are reduced. According to the invention, the restricted passage which connects the working chamber and the discharge port is replaced with a passage that establishes free communication between these ports, and control stability is maintained by providing a dashpot for the motor piston. The adjustable element is so designed that it may overtravel the minimum displacement-establishing position in the displacement decreasing direction and means are provided for venting the working chamber of the motor whenever this overtravel occurs. It has been found that neither the substitution of the dashpot for the low restriction nor the inclusion of the venting means, by itself, produced an appreciable improvement in the oversurge condition but that the use of both schemes was quite effective. With this combination, the magnitude of the pressure surges has been reduced to about 500 p.s.i. above the maximum setting of the pump and the duration of these surges has been reduced to about one second.

The preferred embodiment of the invention will now be described in detail with reference to the accompanying drawings, in which: Y

F1G. 1 is a partial axial sectional view of a swash plate pump incorporating the invention; the displacement controlling mechanism being shown in the maximum displacement-estab]ishing position.

3,016,018 Patented Jan. 9, 1962 rice the displacement controlling mechanism in the oversurge position.

As shown in FIG. 1, the pump comprises a housing having separable sections 11 and 12 which are joined by bolts 13, and which are formed with shoulders for securely clamping and holding a cylinder'block 14. The section 11 is cored to provide an inlet chamber 15 that communicates with inlet port 16 and the section'12 is provided with a discharge port 17 that leads into Vthe annular manifold 18. Cylinder block 14 contains a circular series of cylinder bores 19'which receive the pistons 21. These pistons are reciprocated in a known manner by swash plate 22 and nutating plate 23. Each of the cylinder bores 19 communicates with the inlet port 16 through the passages 24, 25 and 26 and chamber 27 formed in cylinder block 14 and through the inlet chamber 15 contained in housing section 11. Fluid discharged under pressure from the cylinder bores 19 ows into the discharge manifold 18 through spring-biased check valves 28.

Each of the pistons 21 contains an axial boreV 29 that is intersected by a series of radial bleed passages 31, and is encircled by a spill-back valve 32 which cooperates with these passages to control the effective stroke or displacement of the piston. When a spill-back vvalve 32 is in the position shown in FIG. l, it closes the passages 31 for the entire discharge stroke of the piston 21 and thus establishes maximum eifective displacement, i.e., all of the fluid displaced by piston 21 (after closure of passage 24) is forced through check valve 28 and discharged through port 17. As this valve 32 is shifted to the right, it uncovers the passages 31 for progressively longer portions of the discharge stroke thus permitting an increasing portion of the fluid displaced by piston 21 during each discharge stroke to return to the inlet po-rt 16 through axial bore 29, radial passages 31, chamber 27', passage 26, and inlet chamber 15. When' the spill-back valve 32 reaches the position shown in FIG. 2, the passages 31are uncovered for the entire discharge stroke. Therefore, in this position, etective displacement is zero. The spill-back Valves 32 are connected together by a spider 33 which is carried by the rod 34 so that the valves are shifted in unison.

Mounted in an axial bore formed in cylinder block 14 is a bushing 35 containing a stepped axial bore having portions that t the smaller and larger diameter portions 36 and 37, respectively, of the rod 34. Seated on the right end of rod 34 is a spring seat 38 that supports a high-rate compression spring 39. This spring 39 is connected in series with a low-rate spring 41 by a large piston 42 that reciprocates Within a cylinder bore 43 formed in end cap 44. Spring 41 is supported by an adjustable seat 45. Piston 42 is pierced by a series of restricted passages 46 and is, in effect, the piston of a dashpot which damps movement of rod 34.

The rod 34 and the spill-back valves 32 are biased i, to the positions shown in FIG. lby springs 39 and 41 FIG. 2 is a sectional view of a portion of the pump,

in the and are shifted in the opposite direction by a pressure motor 47. This motor includes a Working chamber 48 that is defined by an annular groove formed in bushing 35, and a piston which is deiined by the shoulder 49 formed at the junction of the two rod portions 36 and 37. An unrestricted passage 51establishes free communication between working chamber 48 and the annular discharge manifold 18, and a shallow stopped groove 52 formed in rod portion 37 serves as a vent for working chamber 48 when the rod 34 is in the oversurge position of FIG. 3.

During normal operation, the parts ofthe displacement controlling mechanism assume the positions shown in FIG. 1 and al1 of the uid displaced by each piston 3 21 discharges from port 17. Discharge pressure in manifold 18 is transmitted to Working chamber 48 by passage 51 and therefore, as this pressure increases so too does the force acting on shoulder 49. When this force exceeds the preload in springs 39 and 41, rod 34 shifts to If, at any time, system demand should decrease to j zero at a rate faster 'than that with which the displacement controlling mechanism can keep pace, system pressure will rise above the desired maximum and motor 47 will cause spill-back valves 32 and rod 34 to overtravel 'the zero displacement-establishing position of FIG. 2

and move to the oversurge position of FIG. 3. The end 53 of vent groove 52 is so located relatively to the right edge of bushing 35 thatthis movement of rod 34 past the FIG. 2 position opens progressively communication between Working chamber 48 and inlet port 16 through groove52, space 54, passage 25, chamber 27, passage 26 and inlet chamber 15. This vent path bypasses uid from discharge port 17 to inlet port 16 and dissipates the pressure surge. As disehargepressure reduces to the desired maximum, rod 34 gradually moves back to the FIG. 2 position.

As stated previously, the drawings and description relate only to a preferred-embodiment of the invention. Since many changes lcan be made in the structure of this embodiment without departing from the Vinventive concept, the following claims should provide the sole measure of the scope of the'invention.

What is claimed is:

l. In a pump of the type including a housing containing inlet and discharge ports, an adjustable element associated with the pump and movable in displacement-increasing and decreasing directions for 'varying the displacement of the pump, and a control motor having a working chamber and a piston subject to the pressure in that chamber and connected with the adjustable element for shifting it in the displacement-decreasing direction, the improvement which comprises a passage establishing free communication between the Working chamber and the discharge port; means controlled by movement of the piston for opening a vent connection leading from the working chamber as the piston approaches the limit of its movement in the displacement-decreasing direction; and a dashpot connected with the piston.

2. In combination, a pump having an inlet and an outlet port; an adjustable element associated with the pump :for varying the displacement of the pump, said element ing chamber and the discharge port; means controlled by movement of the piston in displacement-decreasing andA displacement-increasing directions from the minimum displacement-establishing position for progressively connecting and disconnecting, respectively, the Working chamber with the inlet port; and a dashpot connected with the piston.

References Cited in the file of this kpatent UNITED STATES PATENTS Kendrick Apr. 15, 1941 2,238,062 2,778,314 Y Siver Jan. 22, 1957 2,918,012 Lucien Dec, 22. 1959 

